Addition with a cantilevered balcony

[ahunter]

I am designing an addition with a cantilevered balcony and am wondering if it is physically possible. the deck is 24' long with 12' extended past I will be using W12 x 96 (massive) it will be held up with two rows of columns that are 12' apart. my question is if the foundation of the columns are heavy/deep enough will the long span be a problem?
see atacment

 

[Studiot]

Hello ahunter an welcome.

What is your balcony for? a Helipad?

It seems a tad overdesigned compared to what I take to be the existing building wall and its foundation on the left.

 

[ahunter]

underneath the part that isn't cantilevered will be three offices and the balcony will be for employees to take breaks and have our company grill outs. It is a bit overdesigned, and we were going for that look just to be different.

 

[Studiot]

That doesn't explain the difference between what seems to be a simple srip foundation at the base of the (existing?) wall on the left and what appear to be bored pile foundations for the new flat roof.

 

[ahunter]

the new foundaion is just there right now to show where it will be (size not correct). I have calculated the adition to weigh around 82,000 lbs (dead weight) and it will be suported by six (8 5/8 dia x 1/4"wall) columns. with a foundation for each of them. my question is how to caculate what the size of the footings will need to be for this weight and the cantilever.

 

[Studiot]

I have absolutely no idea of the bearing capacity of your soil. Soil capacities vary greatly from less than half a ton per square foot for loose soil to over 100 tons per square foot for solid rock.

Particularly since this project is for acompany and employees you need to get a competent structural engineer, who knows local conditions and codes, to advise.

To get an idea of the foundation size take the weight on any column and divide by its area to get the foot pressure.

Now imagine this spreads out at 45 degrees from this level. As the area increases the load remains the same, so the pressure decreases. At some point this pressure equals to bearing competency of the soil.
This is your foundation level. You will need a block of concrete to fill the space between this level and the foot of the column. The area of the concrete block obviously equals the weight on the column divided by the allowable bearing pressure on your soil.